Present-day nuclear power reactor fuels in use for commercial power generation are based on uranium dioxide. The uranium dioxide fuel is commonly a product of several manufacturing steps including pressing a uranium dioxide powder into a pellet shape and subsequently firing the pellet to remove any formed voids.
The wide-spread use of uranium dioxide fuel is due primarily to the many desirable characteristics of the uranium dioxide material, including a high density of uranium atoms necessary for producing a nuclear reaction, inertness and insolubility of the uranium dioxide in high temperature water, a high melting point and an absence of neutron poisons which could affect reactor performance. Although uranium dioxide is satisfactory for use in light water reactors, uranium dioxide also has several significant drawbacks which limit its overall effectiveness. Chief among the drawbacks is a relatively low thermal conductivity of uranium dioxide which imposes significant limitations on reactor operations. The inability of uranium dioxide to remove large quantities of heat effectively limits overall reactor operation and also compromises reactor operations during transient events such as loss of coolant accidents (LOCA). The nuclear industry has made attempts to increase thermal conductivity of uranium dioxide fuel, but none of the attempts have been successful. Despite the drawbacks, uranium dioxide, in unmodified form, remains the dominant fuel for nuclear power reactors.
In general, heat produced in nuclear fuel must be conducted through the body of the fuel, normally in the pelletized form, and an external cladding, normally a zirconium alloy, to a surrounding coolant layer in order to properly cool the fuel and prevent pellet degradation. The surrounding coolant layer is moved past the external cladding to provide a consistent temperature for removal of heat from the fuel. During transient reactor conditions, such as when the coolant flows past the external cladding unevenly, the steady removal of heat from the pellet is disrupted. During loss-of-coolant accidents, operational safety is compromised due to accumulating heat in the fuel and the inability of the uranium dioxide matrix to withstand the increased temperatures. This thermal conductivity characteristic of conventional uranium dioxide fuel necessitates operating the reactor at reduced power in order to achieve acceptable overall plant safety margins. Operating the reactor at the reduced power levels consequently affects overall plant operating costs. Current nuclear fuels using uranium dioxide also have a limited burn-up capacity. The limited burn-up capacity reduces the overall cost effectiveness of the fuel. The limited burn-up capacity results from greater fission gas release inside the fuel cladding over time. The greater fission gas release thereby results in higher fuel rod internal pressure, potentially leading to cladding deterioration due to the higher stress. The elevated temperatures of the existing fuel also stresses the fuel cladding thereby limiting overall service life.
There is a need to provide a nuclear fuel which will provide enhanced thermal conductivity compared to conventional uranium dioxide fuel currently used in nuclear power reactors.
There is a further need to provide a nuclear fuel which will result in greater safety of the nuclear reactor under accident conditions, such as loss of coolant accidents.
There is a still further need to provide a nuclear fuel which will possess superior burn-up capabilities compared with conventional uranium dioxide nuclear fuels, thereby allowing greater fuel utilization, improved economy, and limited fission gas release.